DC power vs AC power

[Anarchist420]

Why did AC win out over DC when DC is cleaner (less harmonic distortion) and uses less power?

I know that AC can be transmitted through longer thinner wire (so it seems cheaper), but why is that considered worth it when it uses so much more power and heat than DC? Isn't it just better to pay more for a thicker copper wire than to pay for conversion transformers and have the burden of using more power that's not even as clean if you don't use conversion? Also, if there is less harmonic distortion going to your components' power supply, then your components will last longer. I think DC power would be cheaper in the long run, since no need to have conversion modules, and no need to replace fried components, however rare that may be with AC.

DC was invented first, and it was expensive, but some of that had to do with the fact that Thomas Edison was granted a patent, so he kept the price up.

I'm not claiming to be an expert, but I think that DC power would cut down quite a bit on your electric bill in addition to be cleaner.

 

[guyver01]

Read this

http://en.wikipedia.org/wiki/War_of_Currents

The advantage of AC for distributing power over a distance is due to the ease of changing voltages using a transformer. Available power is the product of current × voltage at the load. For a given amount of power, a low voltage requires a higher current and a higher voltage requires a lower current. Since metal conducting wires have an almost fixed electrical resistance, some power will be wasted as heat in the wires. This power loss is given by Joule's laws and is proportional to the square of the current. Thus, if the overall transmitted power is the same, and given the constraints of practical conductor sizes, high-current, low-voltage transmissions will suffer a much greater power loss than low-current, high-voltage ones. This holds whether DC or AC is used.

Converting DC power from one voltage to another requires a large spinning rotary converter or motor-generator set, which was difficult, expensive, inefficient, and required maintenance, whereas with AC the voltage can be changed with simple and efficient transformers that have no moving parts and require very little maintenance. This was the key to the success of the AC system. Modern transmission grids regularly use AC voltages up to 765,000 volts.[12] Power electronic devices such as the mercury arc valve and thyristor made high-voltage direct current transmission practical by improving the reliability and efficiency of conversion between alternating and direct current.

Alternating-current transmission lines have losses that do not occur with direct current. Due to the skin effect, a conductor will have a higher resistance to alternating current than to direct current; the effect is measurable and of practical significance for large conductors carrying thousands of amperes. The increased resistance due to the skin effect can be offset by changing the shape of conductors from a solid core to a braid of many small (isolated) wires. However, total losses in systems using high-voltage transmission and transformers to reduce the voltage are very much lower than DC transmission at working voltage.

 

[zzuupp]

It's all in the transmission costs. It's simple to change up AC voltage for long distance transmission, and then back down to the end user.

 

[Nik]

Tesla, how I love thee.

 

[PottedMeat]

It's all in the transmission costs. It's simple to change up AC voltage for long distance transmission, and then back down to the end user.

there isn't much that's more reliable or more simple than some windings around a core dunked in some cooling fluid - plus the isolation

 

[PieIsAwesome]

Easy voltage step-down.

 

[AyashiKaibutsu]

Because Edison was an idiot and Tesla was a genius that's why : p

 

[Mark R]

The detailed technical description has already been posted.

The short answer is losses.

DC has roughly half the wiring losses of AC, all other things being equal. However, AC can easily have the voltage changed for transmission/use, and this is hugely important for losses, and makes the 50% advantage of DC a total nothing burger.

Increasing the voltage on a line by a factor of 10 can reduce the losses by a factor of 10 while simultaneously permitting the use of 10 times less metal conductor. Transformers are extremely efficient, better than 98% efficient for distribution transformers, and extremely reliable, so the costs (including losses) of transformers are vastly less than a DC system.

Another important issue is that AC motors and generators are much simpler, more efficient, and much more reliable than DC motors/generators.

Harmonic distortion is a feature of AC that is related to non-linear loads. It doesn't occur in DC, as there is no frequency to develop harmonic distortion. The effects on AC systems are relatively modest, however, and mitigation techniques are generally relatively low cost. While harmonic distortion does reduce the efficiency of things like motors, transformers, etc. this is nothing like the reduced efficiency of replacing an AC motor with a DC motor, or using motor/generator setups to convert DC voltages. Harmonic distortion is much less of an issue than surges from inductive loads or lightning discharges - these are equally troublesome on AC and DC supplies.

Converting DC voltages for home appliances/electronic devices, has until relatively recently been difficult. It had always been done traditionally with an AC transformer, to change the voltage, and then a rectifier to convert to DC. Modern technology has now brought advanced DC-DC conversion technologies (SMPS), but these are extremely complex and sensitive electronic devices, requiring exceptionally careful design, and have only really been available in the last 15-20 years or so. Transformers are extremely reliable and extremely simple.

 

[NL5]

Tesla, how I love thee.

Because Edison was an idiot and Tesla was a genius that's why : p

Yep, Tesla.

DC is still used for long distance transmission.

 

[DominionSeraph]

DC is still used for long distance transmission.

Where?

I think undersea cable use DC to power the fiber optics, but that's the only thing I can think of.

 

[LTC8K6]

IIRC, DC is being used for long distance transmission in many places.

http://en.wikipedia.org/wiki/Pacific_DC_Intertie

It's coming back into favor for several reasons on long distance runs.

 

[Anarchist420]

Thanks for the replies guys, I had forgot that current and voltage are codependent. As noted by others, there is of course, ways to get DC quality power without having DC power grids. The DC->DC conversion technology should be used more often, so power output is cleaner (so that it doesn't go back to AC).

As for whether DC is still in use for long transmissions, I believe it is in Japan, but they have to have more DC power grids or something like that.

 

[NL5]

As for whether DC is still in use for long transmissions, I believe it is in Japan, but they have to have more DC power grids or something like that.

Used in the US as well. Over long distances the added cost of Substation gear is outweighed by savings in transmission lines and IsquaredR losses.

 

[JEDI]

Paging Rubycon...

 

[Mark R]

Thanks for the replies guys, I had forgot that current and voltage are codependent. As noted by others, there is of course, ways to get DC quality power without having DC power grids. The DC->DC conversion technology should be used more often, so power output is cleaner (so that it doesn't go back to AC).

As for whether DC is still in use for long transmissions, I believe it is in Japan, but they have to have more DC power grids or something like that.

DC doesn't really have much of an advantage in terms of cleaner power.

Single phase AC has a problem in that the power flow isn't continuous - it shuts off 100 or 120 times per second depending on frequency - lighting may flicker, and heavy motors may vibrate, etc. DC is continuous - but so is 3 phase AC (while one phase is shut off, the other two are working harder). So, in factories with rotating motors where you could get a strobe light effect from flickering lights, 3 phase lighting is used (DC could be used, but AC is much more efficient and practical for fluorescent and other high-efficiency lighting). Similarly 3 phase motors are used because they are extremely smooth (they are also significantly more efficient than single phase AC motors, as well as smaller, cheaper and more reliable).

The advantages of DC transmission are 2 fold:
1. It is 50% more efficient than AC, all other things being equal. So for super-long transmission lines, where losses are greater, the additional energy savings of DC are helpful, where when it becomes impractical to boost the voltage any further, so where extreme voltage lines (750,000 +) would be necessary, DC may be selected.

2. It can connect between 2 AC grids which are not synchronized. In an AC grid, all generators must turn at the same speed, and the generator rotors must be at the same point in rotation (i.e. all generators in the grid must pass the 12 o'clock point simultaneously). It isn't possible to connect directly 2 AC grids that are not held in sync. So, where power needs to be exchanged between unsynchronized grids (e.g. Texas/Eastern US, or UK/France) or between grids of different frequencies (e.g. Tokyo 50 Hz and Kyoto 60 Hz), a DC link is the only practical way.

 

[DrPizza]

It's very clear why AC beat DC in the beginning. In modern times, we've invented ways to more efficiently convert one DC voltage to another. But, switching everything to DC instead of AC would require drastically changing a lot of our infrastructure.

Also, in the beginning, components really didn't give a shit whether it was DC or AC. You really don't think they had electronics back then, do you? It was light bulbs & motors.

 

[Modelworks]

I am a long time follower of Tesla's work. I have all his patents printed, books, notes, letters he wrote to people, etc. One of the things they are studying is the belief by Tesla that DC power and DC circuits went against the natural flow of electricity and that it was a forced method of power where AC and high frequencies allowed nature to take its course and could be used to do work far easier than DC. He compared DC to trying to force current on a selected path whereas AC was providing the path and letting the current flow at its own rate. He wrote a long letter about it. I guess you could compare it to someone going from point A to point B using a train vs someone driving in a car. The test currently being conducted have shown that electricity at very high frequencies starts to behave differently from how physics says it should. As much as he was about electricity , Tesla was more interested in frequencies and harmonics with the belief you could send power wireless anywhere in the world if you could find the right frequencies on both ends.

Everything he designed was based on the concept of letting nature do the work vs trying to force it do the work.

 

[mmntech]

From what I've read, power transmission was the big thing. With DC, each neighborhood required their own generator. Usually coal fired. AC's long distance efficiencies made it possible to locate noisy, dirty generators outside city limits. Or more importantly, near power sources like Niagara Falls.

As inverter technology improves, High Voltage DC transmission is coming more common. They use it in parts of Europe.

 

[jman19]

Tesla, how I love thee.

Just recently, some nutter in the street was complaining to me about how others have made riches off of Tesla's work yet was giving nothing back to "her people"... then she told me that some Jews had a black man punch her in the face to break her orbital bone to ruin her natural, "pure" (from a genetic standpoint) appearance.

Ok I don't know why I posted this, your mention of Tesla reminded me of this incident and I guess I just felt like venting...

 

[Leros]

It would be interesting to see AC to DC converters on a larger level. It makes since for the power companies to put big converters at the level of a block or neighborhood. You could really save on losses due to having a converter on every device.

Granted, most of the energy consuming devices are still AC. However, DC devices are starting to consume more and more energy (high end PCs, large LCDs, gaming consoles, etc).

 

[DominionSeraph]

It would be interesting to see AC to DC converters on a larger level. It makes since for the power companies to put big converters at the level of a block or neighborhood. You could really save on losses due to having a converter on every device.

Granted, most of the energy consuming devices are still AC. However, DC devices are starting to consume more and more energy (high end PCs, large LCDs, gaming consoles, etc).

The problem there is that you then either need to convert the DC voltage to the operating voltage or use a voltage divider. The former is going to be way more expensive than a simple power supply and the latter is going to be horribly wasteful.
Your CPU runs at ~1.3VDC, not 100V. That's easier to get to from AC.

 

[Mark R]

The problem there is that you then either need to convert the DC voltage to the operating voltage or use a voltage divider. The former is going to be way more expensive than a simple power supply and the latter is going to be horribly wasteful.
Your CPU runs at ~1.3VDC, not 100V. That's easier to get to from AC.

That's actually debatable these days, at least if one considers the overall cost. DC-DC converter technology has progressed to the point, where all PSUs for domestic electronic devices are DC-DC converters - mainly because the amount of expensive raw materials (copper and iron) for a DC-DC converter is less than a transformer. Even relatively heavy equipment such as microwave ovens are now using DC-DC converters/inverters instead of transformers, and most new HVAC equipment also uses DC-DC converters/inverters to power the compressor and fans. The point is that AC is great because it is trivially easy to convert it to DC, if that is what you want.

If you've actually looked at a PC PSU - you'll find that the actual PSU itself runs on DC. The first components that the electricity passes through are diode rectifiers to convert the AC to DC (usually about 330-340 V). If you wanted, you could actually connect a PC PSU to a 350 V DC supply, and it would run fine.

Had a look at a modern fluorescent light? You guessed it, there's a DC-DC converter hiding in the fitting (or in the bulb, in the case of CFLs). They still accept AC because of built-in rectifiers, but they'll run fine on DC (and this is a design feature as it permits them to run on a building's battery supply with no additional circuitry, or as direct replacement for emergency incandescent lamps).

While DC-DC converters are now a commodity item, power is still a problem, and the cost of DC-DC converters rises drastically once you go past a few kW - Massive DC-DC converters like the 2000 MW converters used for powerlines are so expensive that DC lines are still rare, except where there are clear benefits. The sheer simplicity, reliability and robustness of transformers (they will handle overloads of 1000% rated capacity for a short time), as well as of AC motors and generators means that AC is still clearly the more practical technology.

 

[Modelworks]

The problem there is that you then either need to convert the DC voltage to the operating voltage or use a voltage divider. The former is going to be way more expensive than a simple power supply and the latter is going to be horribly wasteful.
Your CPU runs at ~1.3VDC, not 100V. That's easier to get to from AC.

Converting it isn't so much an issue. You can use a simple switcher to convert it with near perfect efficiency. The issue with DC for me is the wire sizes required when you start needing to move large currents over a distance. Assume an entire home is wired for 24VDC , that is the voltage most used in UPS that convert to AC as well as solar panels. A typical branch circuit in a home is 20A @ 120VAC for 2400 watts, so at 24VDC we need 100A to equal the same amount of power. To run a 20 foot line from an outlet back to the meter would require a 1 AWG wire and even then you only get 23.5VDC at the outlet. AC can do it with 8AWG and the result would be 117VAC at the outlet.

To compare the wire size, 1AWG is 7.3mm diamter and 8AWG is 3.2mm over half as small. I personally can't imagine wiring a home with 1AWG or powering a 750watt PC system from DC outlets with a power cord that had wires that looked like jumper cables.

 

[PowerEngineer]

What Mark R said... :thumbsup:

 

[edro]

I think the invention of AC is amazing. It is such a revolutionary idea.
The water flow and current flow analogy was so common, it seems like a huge leap to come up with AC.

I think if AC wasn't thought of, DC would have been just as successful and we would probably have much more common power systems (no frequency differences, probably just a few common voltages, etc.).

If Tesla didn't come up with AC, would someone have eventually?
Inventions come in pulses, triggered by society and timing of other recent inventions.
Whatever triggered the thought in Tesla probably would have been triggered in someone else, eventually.